Cis-het is contemporary academic slang for cisgender heterosexual, and cisgender itself is contemporary academic usage for non-transgender.
In Latin, I'm told, the opposite of trans is cis. Well, in some contexts, anyway.
When I was in college, the polite term was transsexual. But that's rude or worse now.
For some reason, most people turn out cis-het.
They'll continue to do so. At least that's my bet.
I do expect these terms to change again.
I guess it's safe to use them until then.